1. Field of the Invention
The present invention relates to a current regulator for LED lighting applications, especially to a cascoded current regulator for LED lighting applications.
2. Description of the Related Art
FIG. 1 illustrates a circuit diagram of a prior art LED lighting circuit. As illustrated in FIG. 1, the prior art LED lighting circuit includes a bridge rectifier 110, an amplifier 120, a current sensing resistor 130, an NMOS transistor 140, and a LED load 150.
The bridge rectifier 110 is used for rectifying an AC power VAC to generate a line voltage VLINE.
The amplifier 120 is used for amplifying the difference of a reference voltage VREF and a feedback signal VFB to generate a gate signal VG, wherein the reference voltage VREF is a DC voltage.
The current sensing resistor 130 is used for generating the feedback signal VFB in response to an output current IO.
The NMOS transistor 140 is used for controlling the output current IO in response to the gate signal VG—the higher the gate signal VG, the larger the output current IO.
The LED load 150, powered by the line voltage VLINE, emits light according to the output current IO—the larger the output current IO, the higher the light intensity.
When in operation, the feedback signal VFB will be regulated at the reference voltage VREF due to a negative feedback mechanism of this circuit, and the drain-source voltage VDS of the NMOS transistor 140 will vary with the line voltage VLINE so as to keep the output current IO constant. However, when the line voltage VLINE is changed from the lowest level to the highest level of an allowed range—for example, the allowed range is 85V˜135V, and the line voltage VLINE is changed from 85V to 135 V—of the prior art LED driver circuit, then the drain-source voltage VDS of the NMOS transistor 140 will increase by 50V, degrading the efficiency of power converted from the line voltage VLINE to the LED load 150, and a large amount of heat will be generated thereby. Besides, the NMOS transistor 140 will have to be manufactured by using a high voltage process.
To solve the foregoing problems, a novel current regulator is therefore needed.